NATURAL HISTORY OF THE SHIP-WORMS. 217 
development are very different from the adult condition. The general plan is 
shown in figure 7, which represents a newly attached larva from the right side, 
with the shell, mantle, and gills removed. A long, ciliated cesophagus (fig. 7, 
22 oe) leads into a rather small stomach, from which projects on each side a large, 
simple, almost spherical liver lobule. The wall of the liver is composed of large, 
coarsely granular, pigmented, nonciliated cells (fig. 7, 23). The intestine leaves 
the right side of the stomach (fig. 7, 24) and after forming a single loop passes 
over the posterior adductor muscle as the rectum. Just posterior to the point of 
origin of the intestine is a small hemispherical diverticulum of the stomach, the 
cecum (ce, fig. 7, 23, 24), composed of densely granular, nonciliated cells. The 
posterior ventral part of the stomach is occupied by the opening of a large conical 
diverticulum, which is median in position, the sheath of the crystalline style (s s, 
fig. 7, 22). Its walls are composed of large, coarsely granular, densely ciliated 
cells characteristic of this structure, except at the blind end, where the cells are 
smaller, more finely granular, and nonciliated (fig. 22). 
The alimentary canal of the larva is interesting because of the advanced 
development of some parts and the retarded development of others. The liver has 
advanced but little in form beyond a stage reached two or three days after hatch- 
ing. On the other hand, the cecum of the stomach, which is peculiar to the 
members of the Pholadacea, is already present as a rudiment, although it is not 
to become functional till after the adoption of the burrowing mode of life in the 
wood. 
As the larva develops into the ship-worm, the size and relations of the parts 
of the alimentary canal change greatly. The csophagus becomes, in the adult, 
very short in comparison with other parts (fig. 10). The stomach elongates pos- 
teriorly more and more (fig, 8, 9, s) till, in the adult, it projects far beyond the 
posterior adductor muscle and forms a long, irregular, more or less cylindrical 
tube (fig. 10). As is well known, the wood grated away in boring is ingested and 
stored in the cecum of the stomach. Even before the ingestion of the wood begins, 
the cxcum projects into the foot as a large hollow vesicle lined by clear, ciliated 
cells; but as soon as wood is ingested it enlarges rapidly and soon forms the larg- 
est part of the digestive system (fig. 8, 9, 10, ce). With its increase in size, it comes 
to leave the posterior end of the stomach and crowds the sheath of the crystalline 
style to the left side (fig. 8, 9, 10). In young specimens the cecum occupies almost 
the whole mass of the foot, and its blind end points forward (fig. 8, 9). As the 
visceral mass elongates, the cecum is gradually drawn backward, till in the adult 
it forms a very long cylindrical tube, stretching to the posterior end of the visceral 
mass (fig. 10, ce). In ship-worms that are boring and growing the cecum is 
always completely filled with ingested particles of wood. The scarcity of diatoms 
and other food materials seems to indicate that in the ship-worm boring an inges- 
tion of wood alternates with ingestion of food, and that in feeding the food is guided 
into the intestines, and in boring the particles of wood into the cecum. The 
cecum of the adult is, then, a long blind tube, opening only at its anterior end 
into the stomach. Internally it is lined with a ciliated mucous membrane, which 
is infolded on the ventral side like a complex typhlosole (fig. 30, 31). This fold 
